Prior field effect transistors (FET) had currents flow only when electric field was applied to a gate, that is, when connected to a power supply, and thus latched switching by a transistor by itself was impossible. Also, even when a drain is applied to a gate of another transistor, result (drain current) of the front transistor is not stored in the subsequent transistor.
Also, prior field effect transistors (FET) had to charge electric charges until a fixed voltage was charged on a gate, and there were difficulties of having to take sufficient time since gate charging time was required. That is, a limit for high-speed operation was reached.
Also, when trying to store calculation results, since calculation results are preserved only when power is connected as with a SRAM, which uses 5 prior field effect transistors (FET), calculating results had to be transferred to a main memory (DRAM or SRAM).
A data line transferring calculation results is called a bus, and it takes a long period of time to send and receive data (calculation results) again through this bus, and this is the reason for requiring a memory in addition to a CPU.